Liquid Crystal Displays (LCD's) have become very common in aircraft cockpits. The LCD's have taken the place of CRTs and other analog instruments which provide the pilot with the information necessary to operate the aircraft. One problem with installing LCD's in a cockpit is the harsh environment in which aircraft operate. In the case of military aircraft, temperatures in which the display must operate can vary from -40.degree. C. to 100.degree. C.
Twisted nematic liquid crystal displays used in the cockpits of aircraft typically include a matrix array of liquid crystal picture elements and a corresponding backlight for illuminating these elements. These pixels are often temperature dependent with respect to their normal operating characteristics in that an LCD relies on the behavioral characteristics of the twisted nematic crystalline layer as it is exposed to driving voltages. When such driving voltages are applied across the liquid crystal material, the nematic liquid crystals tend to align themselves so as to provide a desired image to the viewer. Because such voltage-related behavior of the twisted nematic material is a function of temperature, the overall performance of the corresponding display is temperature dependent. When below a particular temperature, twisted nematic liquid crystal material does not behave in a consistent manner. Accordingly, the LC material in such situations must be heated to a desired temperature in order to achieve satisfactory functionality. The amount of time it takes the LC material to be heated to this level is known as the display's "warm up" period of time.
In order to bring the liquid crystal material up to a desired operating temperature, a cockpit display must include some type of heating element. An integral heater element for an LCD generally comprises a thin film of transparent indium tin oxide (ITO) deposited proximate to the liquid crystal layer of the display. By maintaining an electrical potential connected to one edge of the ITO heater and switchably connecting the opposite side of the heater to ground, the LCD can be selectively heated during cold temperature operation.
When liquid crystal displays operate at a temperature which is too high, they have a tendency to crack. For this reason, it is desirable to mount on the liquid crystal display assembly a temperature sensor which closely monitors the temperature of the liquid crystal material. Because most temperature sensors are opaque, they have always been mounted somewhere on the LCD chassis and not in the viewing area. The sensors are usually mounted on the edges of the liquid crystal display on some kind of support structure. The disadvantage of this placement of the temperature sensor is that the sensors tend to average the LCD temperature with that of the chassis. This results in a thermal gradient greater than 120.degree. C. under extreme temperature conditions.
Therefore, to avoid this type of cracking, a temperature sensor is needed which accurately measures the temperature of the LC material and not that of the display chassis.